The present invention relates generally to devices and methods for determining the visual contrast sensitivity of a subject, and more particularly to a novel vision chart and method for determining the contrast sensitivity of a subject by varying spatial frequency of a target at constant contrast.
The contrast sensitivity function (CSF) is a measure of the contrast required for an individual to resolve (or see) a bar type grating of various spatial frequencies. The standard techniques to measure CSF comprise presentation to a subject a pattern of particular spatial frequency the contrast of which may be varied. Prior art methods and test charts for determining CSF in a vision system may be exemplified by that described in or referenced by U.S. Pat. No. 4,365,873 to Ginsburg, entitled "Spatial Frequency and Contrast Sensitivity Test Chart", and U.S. Pat. No. 4,293,200 to Dobson et al, entitled "Visual Faculty Testing Apparatus".
The present invention describes a novel vision test method and device for rapidly and reliably measuring the contrast sensitivity function of the human vision system, and comprises a chart including a plurality of visual grating type test patches of the same spatial frequency but of different contrasts. In the use of the chart of the invention, the subject observes the chart as it is brought toward the subject, as by the subject walking toward the chart mounted on a wall, until the subject can resolve in turn each grating test patch. Changing the distance at which a fixed linear frequency at various contrasts is displayed provides a display of continuously variable angular spatial frequency at various contrasts. The distance at which the subject can resolve a particular test patch on the chart as displayed is related to the angular spatial frequency observable by the subject. A full range of CSF measurements characterizing the vision system of the subject may thus be made using spatial frequency as the dependent variable for contrast sensitivity determinations. Unlike devices and methods of the prior art which test visual acuity (resolving power) by testing the resolving power of the eye to distinguish sharp edges, the present invention provides means to test the ability of the eye to discern targets of various contrasts as they are presented at various spatial frequencies, by testing the eye's ability to discriminate a cyclical pattern (such as a square wave) from a non-patterned adjacent test patch, both test areas having the same space average reflectance (or luminance). The chart of the present invention therefore is substantially less complicated, contains fewer dissimilar targets, is considerably less expensive to fabricate, and is easier to calibrate and use than charts provided by the prior art.
It is, therefore, a principle object of the present invention to provide a novel and inexpensive method for rapidly and reliably determining contrast sensitivity of a subject.
It is a further object of the invention to provide an improved test chart for determining contrast sensitivity of a subject.
These and other objects of the present invention will become apparent as the detailed description of certain representative embodiments thereof proceeds.